Simulation of Raman spectra of C and C by non-orthogonal tight-binding molecular dynamics

A non-orthogonal tight-binding molecular-dynamics formalism is used to simulate Raman spectra of the fullerene molecules C₆₀ and C₇₀. Two parametrization schemes for the Hamiltonian and the overlap matrix elements are investigated. The considered molecules are excited randomly and the Fourier transform of the displacement autocorrelation function is employed to extract the vibrational properties. Fair agreement with experiment and with force-constant and ab initio calculations is achieved, with comparatively smaller maximum errors in the frequencies than for other molecular dynamics or semi-empirical calculations from the literature. Received 4 February 1999 and Received in final form 28 November 1999     

Ab initio molecular dynamics study on Ag (n = 4, 5, 6)

Ab initio Molecular Dynamics (MD) method, based on density functional theory (DFT) with planewaves and pseudopotentials, was used to study the stability and internal motion in silver cluster Ag_n, with n =4-6. Calculations on the neutral, cationic and anionic silver dimer Ag₂ show that the bond distance and vibrational frequency calculated by DFT are of good quality. Simulations of Ag₄, Ag₅, and Ag₆ in canonical ensemble reveal distinct characteristics and isomerization paths for each cluster. At a temperature of 800 K, an Ag₄ has no definite structure due to internal motion, while for Ag₅ and Ag₆the clusters maintain the planar structure, with atomic rearrangement observed for Ag₅ but not for Ag₆. At a temperature of 200 K, Ag₄ can exist in two planar structures whilst Ag₅ is found to be stable only in the planar form. In contrast Ag₆ is stable in both planar trigonal and 3D pentagonal structures. Micro-canonical MD simulation was performed for all three clusters to obtain the vibrational density of states (DOS). Received 5 May 1999 and Received in final form 20 August 1999     

Large-scale molecular dynamics simulations of high energy cluster impact on diamond surface

Large-scale molecular dynamics simulations with high acceleration energy on a diamond surface were performed in order to investigate the surface erosion process. Accelerated argon or CO₂ clusters (∼960 atoms, 100 keV/cluster) impacted on the (111) surface of diamond which consisted of more than 1,000,000 carbon atoms. A typical hemispherical crater appeared about 0.7 ps after the impact, and two or three-layered shockwaves were formed and propagated to certain directions, but the crater was immediately filled up with the fluidized hot carbon material due to the collective elastic recovery before the reflection of the shockwave. The impact energy of the cluster was at first transferred mainly as kinetic energy of the diamond surface in a short time, and the potential energy was activated later. The activated carbon and oxygen atoms from the impact cluster stimulated the evaporation from the diamond surface for the CO₂ cluster impact while the evaporation seemed to be suppressed by the argon atoms themselves for the argon cluster impact. Received 22 November 2000     

Theoretical investigation of the ultrafast NeNePo spectroscopy of Au 4 and Ag 4 Clusters

Ultrafast ground state nuclear dynamics of Au₄ and Ag₄ is theoretically explored in the framework of negative ion - to neutral - to positive ion (NeNePo) pump-probe spectroscopy based on the ab initio Wigner distribution approach. This involves the preparation of a nonequilibrium neutral ensemble by pump induced photodetachment of a thermal anionic ground state distribution, gradient corrected DFT classical trajectory simulations “on the fly” on the neutral ground state, and detection of the relaxation process of the ensemble in the cationic ground state by a time-delayed probe pulse. In Au₄, the initially prepared linear structure is close to a local minimum of the neutral state giving rise to characteristic vibrations in the signals for probe wavelength near the initial Franck-Condon transition. A timescale of ∼1 ps for the structural relaxation towards the stable rhombic D_2h neutral isomer was determined by the increase of the signal for probe wavelength in vicinity of the vertical ionization energy of the rhombic structure. In contrast, the relaxation dynamics in Ag₄ is characterized by normal mode vibrations since both the initially prepared anionic ground state and the neutral ground state have rhombic minimum geometries. Thus, time-resolved oscillations of pump-probe signals are fingerprints of structural behaviour which can be used experimentally for the identification of particular isomers in the framework of NeNePo spectroscopy. Received 22 December 2000     

Transport properties of a nanotube-based transistor

Transport properties of doped nanotube-based double junctions forming a nanotransistor are investigated within the tight binding formalism. The effects of doping, gate length and gate-source hopping have been considered. It is found that in addition to the importance of rotational symmetry in determining transport properties, large gains can be achieved for semiconducting doped tubes. Received 30 November 2000     

Structural properties and reactivity of bimetallic silver-gold clusters

Bimetallic silver-gold clusters are well suited to study changes in metallic versus ionic properties involving charge transfer as a function of the size and the composition. We present structures, ionization potentials (IP) and vertical detachment energies (VDE) for neutral and charged bimetallic Au_nAg_m ( 2(n + m)5) clusters obtained from density functional level of theory. In the stable structures of these clusters Au atoms assume positions which favor charge transfer from Ag atoms. In clusters with equal numbers of hetero atoms (n = m = 1- 4) heteronuclear bonding is preferred to homonuclear bonding, giving rise to large values of ionization potentials. For larger clusters (n=m=5, 10) stable structures do not favor neither hetero bonding nor segregation into the single components, although they exhibit more metallic than ionic features. This remains valid also for Au₈Ag₁₂ cluster characterized by strong charge transfer to gold subunit. The influence of doping of pure gold clusters with silver atoms on VDE and IP values is discussed in context of their reactivity towards O₂ and CO molecules. As a starting point we consider reactivity towards CO and O₂ molecules on the example of AgAu^- dimer. The results show that the catalytic cycle can be fullfilled.     

Initial growth of single-walled carbon nanotubes on supported iron clusters: a molecular dynamics study

Molecular dynamics simulations were used to study the initial growth of single-walled carbon nanotubes (SWNTs) on a supported iron cluster (Fe₅₀). Statistical analysis shows that the growth direction of SWNTs becomes more perpendicular to the substrate over time due to the weak interaction between carbon nanotube and the substrate. The diameter of the nanotube also increases with the simulation time and approaches the size of the supported iron cluster.     

Ultrafast multidimensional dynamics of strong hydrogen bonds

The laser driven dynamics of the OH(D) stretching vibration in phthalic acid monomethylester is investigated. The combination of a 55-dimensional all-Cartesian reaction surface Hamiltonian and the time-dependent self-consistent field approach is shown to provide a microscopic picture of intramolecular vibrational energy redistribution taking place upon interaction with an external laser field. Choosing suitable zeroth-order vibrational states and combinations thereof a quasi-periodic in-phase and out-of-phase oscillatory behavior is observed manifesting energy flow on different time scales. The fingerprints of this behavior in transient absorption spectroscopy are also discussed. Received 24 August 2000 and Received in final form 11 October 2000     

Theoretical exploration of ultrafast spectroscopy of small clusters

We present the ultrafast multistate nuclear dynamics involving adiabatic and nonadiabatic excited states of non-stoichiometric halide deficient clusters (Na_nF_n-1) characterized by strong ionic bonding and one-excess electron for which the “frozen ionic bonds” approximation has been justified allowing to consider the optical response of the single excess electron in the effective field of the other electrons. We combined the Wigner-Moyal representation of the vibronic density matrix with the ab initio multi state molecular dynamics in the ground and excited electronic states including the nonadiabatic couplings calculated “on the fly” at low computational demand. This method allows the simulation of femtosecond pump-probe and pump-dump signals based on an analytical formulation, which utilizes temperature dependent ground state initial conditions, an ensemble of trajectories carried out on the electronic excited state as well as on the ground state after the passage through the conical intersection in the case of nonadiabatic dynamics and for probing either in the cationic state or in the ground state. The choice of the systems we presented has been made in order to determine the timescales of the fast geometric relaxation leaving the bonding frame intact as during the dynamics in the first excited state of Na₄F₃, and of the bond breaking processes leading to conical intersection between the first excited state and the ground state as in Na₃F₂. The former is the smallest finite system prototype for an surface F-center of bulk color centers. The latter allows to study the photo isomerization in full complexity taking into account all degrees of freedom. In the case of Na₄F₃ after the fast geometric relaxation in the excited state leading to deformed cuboidal structure without breaking of bonds, different types of internal vibrational redistribution (IVR) processes have been identified in pump-dump signals by tuning the dump laser. In contrast, from the analysis of the pump-probe signals of Na₃F₂ cluster, the timescales for the metallic and the ionic bond breaking, as well as for the passage through conical intersection have been determined. Finally the conditions under which these processes can be experimentally observed have been identified. Received 22 December 2000     

Dynamical aspects and the role of IVR for the reactivity of noble metal clusters towards molecular oxygen

Here, we present the dynamical aspects and the role of internal energy redistribution (IVR) in the reactivity of noble metal clusters towards O₂. We show on the example of Ag₃O₂ ^- / Ag₃O₂ / Ag₃O₂ ⁺ that NeNePo spectroscopy carried out under zero electron kinetic energy (ZEKE) conditions can be a powerful tool to investigate the geometry relaxation and IVR induced by photodetachment in real time. Furthermore, we demonstrate that difference in the reactivity of Ag₆ ^- and Au₆ ^- towards O₂ can be attributed to different nature of the IVR process. Dissipative IVR in Ag₆ ^- favors fast complex stabilization, whereas resonant IVR found for Au₆ ^- might be an important factor determining the catalytic activity of Au₆ ^- cluster in the CO oxidation.     

Kinetics and equilibrium of small metallic clusters: Ab initio confinement molecular dynamics study of 4

The ab initio molecular dynamics (AIMD) [1] is combined with the heuristic, successive confinement method of surveying a potential energy surface (PES) [2], thereby offering a framework for the simulation study of kinetics and equilibrium properties of metallic clusters. This approach is applied to the study of Au₄, a cluster possessing a simple but specific PES, which consists of very shallow and deep basins and due to this presents a challenge to the conventional AIMD methods. Among other things, the probabilities of the transitions between isomers have been found, and on this basis, both the time-dependent and equilibrium populations of the isomers have been calculated for the conditions typical of the NeNePo experiments [3] in the femtosecond pump-probe spectroscopy.     

$ {\rm{V}}_2 {\rm{O}}_5^ + $ {\rm{V}}_2 {\rm{O}}_5^ + " border="0"> reaction with C2H4: theoretical considerations of experimental findings

Density functional calculations have been performed for the reactions towards ethylene considering atomic and molecular oxygen loss, oxygen transfer and association reactions. The oxygen transfer channel to ethylene is energetically favourable in contrast to the oxygen loss. This is in agreement with the experimental results [1] which show that does not lose atomic oxygen during the collision induced dissociation at thermal energies. A radical cation mechanism based on structure-reactivity relation of cluster is proposed to explain oxygen transfer channel indicating that this reaction is size selective.     

DFT investigation of the geometrical and electronic structures of (X = B, N and Si) clusters

Geometrical and electronic structures of C₃₅X fullerenes with , N and Si as substitutional dopants have been studied. Three non-equivalent sites in the D_6h structure of C₃₆ have been considered for the substitution. We have found that the dopant has a strong tendency to substitute at sites where the carbon atom contributes significantly to the frontier orbitals of C₃₆ and has the weakest interaction with its nearest-neighbor atoms. The relative stability of C₃₅Si and C₃₅B (C₃₅N) has been investigated and high chemical reactivity of C₃₅Si has been predicted. Received 8 July 1999 and Received in final form 4 October 1999     

Study of oxygen-C 60 compound formation by NEXAFS and RIXS

The interaction of oxygen with C₆₀ molecules was studied on a C₆₀ film which had been exposed simultaneously to oxygen and UV-light for 190 hours, producing an approximately C₆₀O₁ stoichiometry in the bulk of the sample. C K-edge and O K-edge NEXAFS (using total fluorescence yield detection) and resonant X-ray inelastic scattering (RIXS) spectra from the sample film were measured and the C K-edge data were compared to the spectra from pristine C₆₀ as reference. The C K-edge absorption and emission spectral profiles of the oxygen-doped sample are similar to those of the C₆₀ reference, suggesting that cage breaking of C₆₀ under these conditions, if any, is negligible. However, the redistribution of intensities in the spectra indicates changes in the occupancies of different molecular orbitals, possibly due to changes in electron density upon reaction. Similarities of the O K-edge soft X-ray emission (SXES) spectra to several small oxygen-containing molecules is being discussed in terms of bonding models. Received 4 December 2000     

Optimized 3D structures and energy bands of peanut-shaped C60 polymers

Optimized three-dimensional (3D) cell structures and energy bands of fused (peanut-shaped) C₆₀ polymers (p55 and p66) have been investigated using the first-principles pseudopotential approach within the local density approximation of the density functional theory. We found that the resulting electronic structure is either metal or semiconductor depending on the shape of the polymer chains and the unit cell structure.     

Dispersion and thermal resistivity in silicon nanofilms by molecular dynamics

On nanoscale, thermal conduction is affected by system size. The reasons are increased phonon scattering and changes in phonon group velocity. In this paper, the in-plane thermal resistivity of nanoscale silicon thin films is analyzed by molecular dynamics (MD) techniques. Modifications to the dispersion relation are calculated directly with MD methods at high temperature. The results indicate that the dispersion relation starts to change for very thin films, at around two nanometers. The reasons are band folding and phonon confinement. Thermal resistivity is analyzed by the direct non-equilibrium method, and the results are compared to kinetic theory with modified dispersion relations. Thermal resistivity is affected by both surface scattering and dispersion. Moreover, in thin films, the characteristic vibrational frequency decreases, which in standard anharmonic scattering models indicates a longer relaxation time and affects the resistivity. The results indicate that in very thin films, the resistivity becomes highly anisotropic due to differences in surface scattering. In two cases, surface scattering was found to be the most important mechanism for increasing thermal resistivity, while in one case, phonon confinement was found to increase resistivity more than surface scattering.     

Fragmentation of water clusters: Molecular-dynamics simulation study

The fragmentation of water clusters, [(H ₂ O)_n;n = 2-8], have been investigated by using molecular-dynamics simulation method. In the simulations a polarizable-dissociable potential energy function for water has been used. Particular attention has bee paid to investigate the effect of structural properties and cluster size on the fragmentation. Received 27 April 2000 and Received in final form 6 October 2000     

A multispectrum fitting procedure to deduce molecular line parameters: Application to the 3-0 band of 12 C 16 O

To deduce accurate infrared molecular line parameters (positions at zero pressure, pressure-shifting and pressure-broadening coefficients, collisional narrowing coefficients, and intensities) from rovibrational spectra, an automatic method based upon a multispectrum fitting procedure has been set up, able to treat simultaneously several laboratory Fourier transform spectra. A validation of this method, using absorption spectra of the 3-0 vibrational band of CO around 6 350 cm^-1, already used to measure line intensities and self-broadening coefficients, is presented, and the advantages of the method are pointed out. The self-collisional narrowing of CO was observed and determined for the first time in Fourier transform spectra: β = 0.028±0.004 cm^-1 atm^-1 at about 296 K. Received 21 September 2000 and Received in final form 15 January 2001     

SiC bonding in ( C 60 ) n Si m clusters

This paper deals with a new type of SiC bonding where silicon atom seems to bridge C₆₀ molecules. We have studied films obtained by deposition of (C₆₀)_nSi_m clusters prepared in a laser vaporization source. Prior deposition, free ionized clusters were studied in a time-of-flight mass spectrometer. Mixed clusters (C₆₀)_nSi_m were clearly observed. Abundance and photofragmentation mass spectroscopies revealed the relatively high stability of the (C₆₀)_nSi _n ⁺ , (C₆₀)_nSi _{n - 1} ⁺ and (C₆₀)_nSi _{n - 2} ⁺ species. This observation is in favor of the arrangement of these complexes as polymers where the C₆₀ cages may be bridged by a silicon atom. Free neutral clusters are then deposited onto substrate making up a nanogranular thin film (≃ 100 nm). The film is probed by Auger and X-ray photoemission spectroscopies, but above all by surface enhanced Raman scattering. The results suggest an unusual chemical bonding between silicon and carbon and the environment of the silicon atom is expected to be totally different from the sp³ lattice: ten or twelve carbon neighbors might surround silicon atom. The bonding is discussed to the light of the so-called fullerene polymerization as observed for pure fullerite upon laser irradiation. This opens a new route for bridging C₆₀ molecules together with an appreciable energy bonding, since the usual van der Waals bonding in fullerite could be replaced by an ionocovalent bond. Such an assumption must be checked in the future by XAS and EXAFS experiments. Received 15 November 2000     

Mechanisms of amorphization-induced swelling in silicon carbide: the molecular dynamics answer

We present here the continuation of an investigation of the irradiation-induced swelling of SiC using classical molecular dynamics (CMD) simulations. Heavy ion irradiation has been assumed to affect the material in two successive steps (a) creation of local atomic disorder, modeled by the introduction of extended amorphous areas with various sizes and shapes in a crystalline SiC sample at constant volume (b) induced swelling, determined through relaxation using Molecular Dynamics at constant pressure. This swelling has been computed as a function of the amorphous fraction introduced. Two different definitions of the amorphous fraction were introduced to enable meaningful comparisons of our calculations with experiments and elastic modeling. One definition based on the displacements relative to the ideal lattice positions was used to compare the CMD results with data from experiments combining ion implantations and channeled Rutherford Backscattering analyses. A second definition based on atomic coordination was used to compare the CMD results to those yielded by a simplified elastic model. The results obtained are as follows. On the one hand, comparison of the swelling obtained as a function of the lattice amorphous fraction with the experimental results shows that the melting-quench amorphization simulates the best the irradiation-induced amorphization observed experimentally. This is consistent with the thermal spike phenomenon taking place during ion implantation. On the other hand, disorder analysis at the atomic scale confirms the elastic behavior of the amorphization-induced swelling, in agreement with the comparison with the results of an elastic model. First, no major structural reconstruction occurs during relaxation or annealing. Second, the systems with the most disordered and constrained amorphous area undergo the largest swelling. This means that the disorder and the constraints of the bulk amorphous area are the driving forces for the swelling observed. On the contrary, the nature of the interface does not affect significantly the swelling observed.